US 4225875 A
A short channel MOS transistor and the method for fabricating same is described wherein the dopant concentrations of the source and drain regions are maintained at different levels of conductivity modifiers. The method described teaches first doping the source region while maintaining the drain region masked and then doping both the source and drain regions.
1. In an improved semiconductor device having a body of semiconductor material of a first conductivity type; first and second regions of an opposite conductivity type sharing a common boundary surface on the body of semiconductor material; a channel region of the first conductivity type, intermediate the first and second regions and sharing the common boundary surface; a layer of gate insulator positioned over the channel region; and a layer of conductive material positioned solely over the channel region on the layer of gate insulator, the layer of conductive material and gate insulator defining a gate member; the improvement comprising;
discrete ohmic connections to the gate member, the first region and the second region;
the first and second opposite conductivity regions having the same type of conductivity modifiers therein; and
the first opposite conductivity region having a concentration of conductivity modifiers therein about two orders of magnitude higher than the second opposite conductivity type region.
2. The improved semiconductor of claim 1, wherein:
the first and second opposite conductivity regions are source and drain regions.
3. The improved semiconductor of claim 1, wherein:
the body of semiconductor material is deposited on an insulative substrate, the substrate being selected from the group consisting of sapphire, spinel and monocrystalline beryllium oxide.
4. The improved semiconductor of claim 3, wherein:
the first and second opposite conductivity regions are source and drain regions.
5. The improved semiconductor of claim 4 wherein the concentration of conductivity modifiers in the source region is about two orders of magnitude higher than that of the drain region.
This invention relates to MOS devices and more particularly to a novel short channel MOS device and the method of fabricating the same.
In my copending application entitled "METHOD OF MAKING NARROW SILICON LINES AND SEMICONDUCTOR DEVICES MADE WITH SUCH LINES", Ser. No. 852,931, filed Nov. 18, 1977 and assigned to the same assignee as the subject application, I describe a short channel semiconductor device and the method of fabricating it. That application is incorporated herein, in its entirety, by reference.
The above mentioned application is directed to short or narrow gates formed by doped polycrystalline silicon (polysilicon) lines formed by the controlled doping of only those exposed edges of a layer of polysilicon. While the described process does, in fact, produce shorter channel regions than heretofore possible with standard photolithographic techniques, it has been found that the maximum operating voltage of such devices is significantly low. In certain instances, this is highly desirable, as for example, where power and heat dissipation considerations dictate the use of low current and low voltage. One rationale for explaining the lower operating voltage resides in the fact that the maximum operating voltage is a function of the source-to-drain spacing. As this spacing is reduced (made shorter) the maximum operating voltage is concomitantly reduced.
However, there are many desirable uses for short channel devices having high operating voltages and an obvious attempt at a solution for achieving such higher voltages would be to reduce the doping concentration of both the source and drain regions. This would be accomplished quite easily since, traditionally, both the source and drain regions are doped simultaneously. Such a solution would then allow the drain-substrate depletion region (the band that extends around the drain region at the substrate) to extend into both the gate and drain regions. The resultant increase in source-to-drain punch-through voltage will be accompanied by a correspondingly increased operating voltage. Since the source and drain regions are doped to the same level, the overall speed of the device will be significantly reduced due to the increased "R-C" (resistance-capacitance) time constance of the device.
In accordance with the teachings of my invention, a short channel device and the method for fabricating the device is described wherein the concentrations of conductivity modifiers in the source and drain regions are maintained at different levels. In the described embodiment, the source region is defined and doped to a first level while maintaining the drain region masked and, therefore, undoped. Thereafter, the mask covering the drain region is removed and both the source and drain regions are doped. The net result is a device having a source region doped to a considerably higher level than the drain region.
FIGS. 1, 2 and 3A are sectional views illustrating the steps of carrying out the method of the present invention.
FIG 3B is a partial plan view of FIG. 3A.
FIG. 4 is a sectional view of the short-channel MOS device made in accordance with the method of the present invention.
Referring now to FIG. 1, there is herein presented one method of fabricating a silicon-on-sapphire short channel, MOS device embodying the subject invention. While the following exegesis will be presented in terms of a silicon-on-sapphire device, it will be obvious to those skilled in the art that while body 12 will be referred to as sapphire other insulative substrates such as spinel or monocrystalline beryllium oxide may be used with no deleterious effects. Further, while body 12 may be a substrate of insulating material it should also be obvious that it may be either a semiconductor material or a layer of a semiconductor material on a substrate, depending on the device being made. Additionally, while there will be described the fabrication of an N channel device, I do not wish to be so limited since it is readily obvious that a P channel device may also be fabricated using the teachings of my invention.
The construction of the configuration of structure 10 is described in detail in my copending application. Briefly, a layer of monocrystalline silicon is deposited on surface 14 of body 12 using any one of many well known techniques one of which is the thermal decomposition of silane in a hydrogen carrier. After the deposition of the polysilicon layer, the layer is masked to expose certain areas and the exposed areas are then etched down to the substrate with the unetched areas forming island 16. Thereafter, island 16 is provided with a layer of dielectric or dielectrics 24 which may be formed either by the oxidation of the surface of silicon island 16 or by the thermal decomposition of silane in an oxidizing atmosphere. Island 16 is next provided with a polysilicon layer (over layer 24) followed by a layer 20 of masking oxide, utilizing any one of many well known techniques. Layer 20 is then provided with a patterned layer of photoresist (not shown) and the exposed portion of masking oxide layer 20 is etched down to monocrystalline silicon layer 18 in order to allow the remaining, unetched portions of layer 20 to act as a mask for the subsequent etching of layer 18. At this point, all exposed portions of polysilicon layer 18 are removed by subjecting structure 10 to a buffered potassium hydroxide etchant which, not only removes the exposed polysilicon but will generally etch under the edges of masking layer 20. These techniques are also well known.
Structure 10 is then subjected to a P-type dopant and heated. The particular dopant used for the P-type dopant is boron in the form of diborane. As indicated by arrows 22, the dopant gas will contact only the exposed edges of polysilicon layer 18 so that the dopant will diffuse into layer 18 laterally along the polysilicon layer, from the exposed edges inwardly thereof. This procedure forms a P-type doped polysilicon strip 18.1 along each of the exposed edges of polysilicon layer 18. Since the diffusion constant of polysilicon layer 18 is well known, the diffusion of the dopant into layer 18 may be accurately controlled by monitoring the time and temperature of the diffusion. Accordingly, the width of the resultant doped polysilicon strips 18.1 can be accurately controlled to provide narrow widths.
As shown in FIG. 2 doped strips 18.1 appear wherever the exposed edges of polysilicon layer 18 reacted with the boron dopant. In this particular embodiment, I have chosen to form a closed loop gate (as shown in FIG. 3B) although other configurations will now become obvious. As shown in FIG. 2, after the formation of doped strips 18.1, masking silicon dioxide layer 20 is removed using a buffered hydrogen fluoride solution to expose the doped (18.1) and the undoped (18) portions of the polysilicon layer. Using layer 18 and strips 18.1 as a mask, the device is now subjected to a phosphorous implant, as indicated by arrows 26 to delineate one edge of the channel and form the source region 16.1. An implantation of phosphorous ions at a dose of 1×1016 ions per cm2 at 50 Kev results in a concentration of about 1020 /cm3 of conductivity modifiers in source region 16.1.
Referring now to FIGS. 3A and 3B, the next step is to remove undoped polysilicon 18 which may be done by subjecting device 10 to a suitable etching solution until such time as all undoped polysilicon is removed. Such solvents are described in U.S. Pat. No. 3,738,880 and include aqueous hydryzine or potassium hydroxide-propanol solutions and the like. The use of any of these solutions will selectively remove only the undoped portion of polysilicon leaving doped polysilicon strips 18.1 unaffected. Thereafter, as shown by arrows 28, the entire device is subjected to another phosphorous ion implantation at a dose of about 1×1014 ions per cm2 at 50 Kev resulting in the completion of the channel region and the formation of drain region 16.2 having a concentration of 1018 /cm3 conductivity modifiers therein. The source region 16.1 will now have a concentration of about 1.01×1020 /cm3 conductivity modifiers as a result of the second implant. This represents an increase in conductivity modifiers, in the source region, of about two (2) orders of magnitude greater than that of the drain region.
Thereafter, as shown in FIG. 4, the device is completed by the formation of field oxide 30 in a well known manner followed by the formation of contact openings and the deposition of metallic contacts 32, 34 and 36 in ohmic contact with source 16.1, drain 16.2 and gate 18.1 respectively.
There is thus provided a novel short channel MOS device wherein the source region is provided with a higher concentration of conductivity modifiers therein than the drain region.